Exhaust purification device of internal combustion engine

ABSTRACT

A NO x  absorbent (17) is disposed in an exhaust passage of an internal combustion engine. This NO x  absorbent (17) absorbs the NO x  when the air-fuel ratio of the exhaust gas flowing into the NO x  absorbent (17) is lean and releases the absorbed NO x  when the air-fuel ratio of the exhaust gas flowing into the NO x  absorbent (17) becomes rich. It estimates the amount of NO x  absorbed in the NO x  absorbent (17) from the engine load and the engine rotational speed and when the amount of the estimated NO x  becomes the maximum NO x  absorption capacity of the NO x  absorbent (17), makes the air-fuel ratio of the exhaust gas flowing into the NO x  absorbent (17) rich.

TECHNICAL FIELD

The present invention relates to an exhaust purification device of an internal combustion engine.

BACKGROUND ART

A diesel engine in which an engine exhaust passage is branched to a pair of exhaust branch passages for purifying NO_(x) in the diesel engine, a switching valve is disposed at the branched portion of these exhaust branch passages, the switching valve is switched each time a predetermined time passes to alternately guide the exhaust gas to one of the exhaust branch passages, and a catalyst which can oxidize and absorb the NO_(x) is disposed in each of the exhaust branch passages is well known (refer to Japanese Unexamined Patent Publication No. 62-106826). In this diesel engine, the NO_(x) in the exhaust gas introduced into one exhaust branch passage is oxidized and absorbed by the catalyst disposed in that exhaust branch passage. During this time, the inflow of the exhaust gas to the other exhaust branch passage is stopped and, at the same time, a gaseous reducing agent is fed into this exhaust branch passage. The NO_(x) accumulated in the catalyst disposed in this exhaust branch passage is reduced by this reducing agent. Subsequently, after the elapse of a predetermined time, the introduction of the exhaust gas to the exhaust branch passage to which the exhaust gas had been introduced heretofore is stopped by the switching function of the switching valve, and the introduction of the exhaust gas to the exhaust branch passage to which the introduction of the exhaust gas had been stopped heretofore is started again. That is, in this diesel engine, seen from the viewpoint of each of the exhaust branch passages, exhaust gas is made to flow for a predetermined time during which the NO_(x) in the exhaust gas is oxidized and absorbed by the catalyst, then the inflow of exhaust gas is stopped for a predetermined period and a reducing agent is fed, whereby the NO_(x) accumulated in the catalyst is reduced.

However, the amount of the NO_(x) which is discharged from the engine changes depending on the operating condition of the engine and therefore the amount of the NO_(x) which is oxidized and absorbed by the catalyst during the predetermined time when the exhaust gas is flowing changes depending on the operating state of the engine during that period. Accordingly, there is the problem that when an engine operating condition under which a large amount of NO_(x) is discharged continues, the NO_(x) oxidizing and absorbing ability of the catalyst ends up becoming saturated during the predetermined time in which the exhaust gas flows and as a result the NO_(x) can no longer be oxidized and absorbed by the NO_(x) absorbent, so the NO_(x) is released into the atmosphere.

As opposed to this, when an engine operating condition in which a small amount of NO_(x) is discharged continues, only a small amount of NO_(x) is oxidized and absorbed in the predetermined time in which the exhaust gas flows. Accordingly, in this case, when the inflow of the exhaust gas is stopped and the reducing agent is fed, only part of the reducing agent is used for the reduction of the NO_(x) and the reducing agent becomes in excess, resulting in the problem of the release of this excess reducing agent into the atmosphere.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide an exhaust purification device which can reduce well the harmful components released into the atmosphere regardless of the magnitude of the amount of the NO_(x) discharged from the engine.

According to the present invention, there is provided an exhaust purification device of an internal combustion engine which has in an engine exhaust passage a NO_(x) absorbent which absorbs the NO_(x) when the air-fuel ratio of the inflowing exhaust gas is lean and which releases the absorbed NO_(x) when the oxygen concentration in the inflowing exhaust gas is reduced and which is provided with a NO_(x) estimating means for estimating the amount of the NO_(x) absorbed by the NO_(x) absorbent and a NO_(x) releasing means for reducing the oxygen concentration in the exhaust gas flowing into the NO_(x) absorbent and releasing NO_(x) from the NO_(x) absorbent when the amount of the NO_(x) estimated to be absorbed in the NO_(x) absorbent by the NO_(x) estimating means exceeds a predetermined allowable value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall view of an internal combustion engine;

FIG. 2 is a diagram showing a map of a basic fuel injection time;

FIG. 3 is a diagram showing a correction coefficient K;

FIG. 4 is a graph schematically showing the concentration of unburnt HC, CO, and oxygen in the exhaust gas discharged from the engine;

FIG. 5 is a diagram for explaining an absorption and release function of the NO_(x) ;

FIG. 6 is a diagram showing the amount of NO_(x) discharged from the engine,

FIG. 7 is a graph showing the capacity of absorption of NO_(x) of the NO_(x) absorbent;

FIG. 8 is a graph showing the characteristics of release of NO_(x) ;

FIG. 9 is a graph showing the change in the correction coefficient K;

FIG. 10 is a graph showing C₁, C₂, α, and β;

FIG. 11 is a diagram showing a map of the exhaust gas temperature T;

FIG. 12 and FIG. 13 are flow charts showing a time interruption routine;

FIG. 14 is a flow chart for calculating the fuel injection time TAU;

FIG. 15 and FIG. 16 are flow charts showing a time interruption routine of another embodiment;

FIG. 17 to FIG. 19 are a flow chart for calculating the fuel injection time TAU of another embodiment;

FIG. 20 is a graph showing the correction coefficient K_(t) ;

FIG. 21 is an overall view of an internal combustion engine showing another embodiment;

FIG. 22 is an overall view of an internal combustion engine showing still another embodiment;

FIG. 23 is a graph showing the amount of NO_(x) discharged from the engine;

FIG. 24 is a graph showing the NO_(x) absorption characteristic of a NO_(x) absorbent;

FIG. 25 is a graph showing the residual rate of the NO_(x) remaining in the NO_(x) absorbent;

FIG. 26 is a flow chart for controlling release of NO_(x) ;

FIG. 27 is an overall view of a diesel engine showing still another embodiment; and

FIG. 28 to FIG. 30 are a flow chart showing the control of NO_(x) release.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows a case where the present invention is applied to a gasoline engine.

Referring to FIG. 1, 1 denotes an engine body; 2, a piston; 3, a combustion chamber; 4, a spark plug; 5, an intake valve; 6, an intake port; 7, an exhaust valve; and 8, an exhaust port, respectively. The intake port 6 is connected to a surge tank 10 via a corresponding branch pipe 9, and a fuel injector 11 injecting the fuel toward the interior of the intake port 6 is attached to each branch pipe 9. The surge tank 10 is connected to an air cleaner 13 via an intake duct 12, and a throttle valve 14 is disposed in the intake duct 12. On the other hand, the exhaust port 8 is connected via an exhaust manifold 15 and an exhaust pipe 16 to a casing 18 including a NO_(x) absorbent 17.

An electronic control unit 30 comprises a digital computer and is provided with a ROM (read only memory) 32, a RAM (random access memory) 33, a CPU (microprocessor) 34, an input port 35, and an output port 36, which are interconnected by a bidirectional bus 31. In the surge tank 10 is mounted a pressure sensor 19 for generating an output voltage proportional to the absolute pressure in the surge tank 10. The output voltage of this pressure sensor 19 is input through an AD converter 37 to the input port 35. Further, the throttle valve 14 has attached to it an idle switch 20 which detects when the throttle valve 14 is open to the idling state. The output signal of this idle switch 20 is input to the input port 35.

On the other hand, a crankshaft 21 has connected to it, for example, an automatic transmission 22. This automatic transmission 22 has attached to it a gear position detector 23 for detecting the position of the transmission gears and a vehicle speed sensor 24 for detecting the speed of the vehicle. The output signals of the gear position detector 23 and the vehicle speed sensor 24 are input to the input port 35. Further, in the exhaust pipe 16 upstream of the casing 18 there is attached a temperature sensor 25 for generating an output voltage proportional to the temperature of the exhaust gas. The output voltage of the temperature sensor 25 is input through an AD converter 38 to the input port 35. Further, the input port 35 has connected to it a rotational speed sensor 26 for generating an output pulse expressing the engine rotational speed. On the other hand, the output port 36 is connected through a corresponding drive circuit 39 to the respective spark plug 4 and fuel injector 11.

In the internal combustion engine shown in FIG. 1, the fuel injection time TAU is calculated based on for example the following equation.

    TAU=TP·K

where, TP is a basic fuel injection time, and K is a correction coefficient. The basic fuel injection time TP shows the fuel injection time necessary for bringing the air-fuel ratio of an air-fuel mixture fed into the engine cylinder to the stoichiometric air-fuel ratio. This basic fuel injection time TP is found in advance by experiments and is stored in advance in the ROM 32 in the form of a map as shown in FIG. 2 as the function of the absolute pressure PM in the surge tank 10 and the engine rotational speed N. The correction coefficient K is a coefficient for controlling the air-fuel ratio of the air-fuel mixture fed into the engine cylinder, and if K=1.0, the air-fuel mixture fed into the engine cylinder becomes the stoichiometric air-fuel ratio. Contrary to this, when K becomes smaller than 1.0, the air-fuel ratio of the air-fuel mixture fed into the engine cylinder becomes larger than the stoichiometric air-fuel ratio, that is, becomes lean, and when K becomes larger than 1.0, the air-fuel ratio of the air-fuel mixture fed into the engine cylinder becomes smaller than the stoichiometric air-fuel ratio, that is, becomes rich.

The value of this correction coefficient K is predetermined in relation to the absolute pressure PM in the surge tank 10 and the engine rotational speed N. FIG. 3 shows an embodiment of the value of the correction coefficient K. In the embodiment shown in FIG. 3, in the region where the absolute pressure PM in the surge tank 10 is relatively low, that is, in the engine low and medium load operation region, the value of the correction coefficient K is made a value smaller than 1.0, therefore at this time the air-fuel ratio of the air-fuel mixture fed into the engine cylinder is made lean. On the other hand, in the region where the absolute pressure PM in the surge tank 10 is relatively high, that is, in the engine high load operation region, the value of the correction coefficient is made 1.0. Accordingly, at this time, the air-fuel ratio of the air-fuel mixture fed into the engine cylinder is made the stoichiometric air-fuel ratio. Further, in the region where the absolute pressure PM in the surge tank 10 becomes the highest, that is, in the engine full load operation region, the value of the correction coefficient is made a value larger than 1.0. Therefore, at this time, the air-fuel ratio of the air-fuel mixture fed into the engine cylinder is made rich. An internal combustion engine is usually operated most frequently with a low and medium load and therefore for the majority of the period of operation a lean air-fuel mixture is burned.

FIG. 4 schematically shows the concentration of representative components in the exhaust gas discharged from the combustion chamber 3. As seen from FIG. 4, the concentration of the unburnt HC and CO in the exhaust gas discharged from the combustion chamber 3 is increased as the air-fuel ratio of the air-fuel mixture fed into the combustion chamber 3 becomes richer, and the concentration of the oxygen O₂ in the exhaust gas discharged from the combustion chamber 3 is increased as the air-fuel ratio of the air-fuel mixture fed into the combustion chamber 3 becomes leaner.

The NO_(x) absorbent 17 contained in the casing 18 uses, for example, alumina as a carrier. On this carrier, at least one substance selected from alkali metals, for example, potassium K, sodium Na, lithium Li, and cesium Cs; alkali earths, for example, barium Ba and calcium Ca; and rare earths, for example, lanthanum La and yttrium Y and a precious metal such as platinum Pt are carried. When referring to the ratio between the air and fuel (hydrocarbons) fed into the intake passage of the engine and the exhaust passage upstream of the NO_(x) absorbent 17 as the air-fuel ratio of the inflowing exhaust gas flowing into the NO_(x) absorbent 17, this NO_(x) absorbent 17 performs the absorption and releasing function of NO_(x) by absorbing the NO_(x) when the air-fuel ratio of the inflowing exhaust gas is lean, while releasing the absorbed NO_(x) when the concentration of oxygen in the inflowing exhaust gas falls. Note that, where the fuel (hydrocarbons) or air is not fed into the exhaust passage upstream of the NO_(x) absorbent 17, the air-fuel ratio of the inflowing exhaust gas coincides with the air-fuel ratio of the air-fuel mixture fed into the combustion chamber 3, and accordingly in this case, the NO_(x) absorbent 17 absorbs the NO_(x) when the air-fuel ratio of the air-fuel mixture fed into the combustion chamber 3 is lean and releases the absorbed NO_(x) when the concentration of oxygen in the air-fuel mixture fed into the combustion chamber 3 is lowered.

When the above-mentioned NO_(x) absorbent 17 is disposed in the exhaust passage of the engine, this NO_(x) absorbent 17 actually performs the absorption and releasing function of NO_(x), but there are areas of the exact mechanism of this absorption and releasing function which are not clear. However, it can be considered that this absorption and releasing function is conducted by the mechanism as shown in FIG. 5. This mechanism will be explained by using as an example a case where platinum Pt and barium Ba are carried on the carrier, but a similar mechanism is obtained even if another precious metal, alkali metal, alkali earth, or rare earth is used.

Namely, when the inflowing exhaust gas becomes considerably lean, the concentration of oxygen in the inflowing exhaust gas is greatly increased. As shown in FIG. 5(A), the oxygen O₂ is deposited on the surface of the platinum Pt in the form of O₂ ⁻ or O²⁻. On the other hand, the NO in the inflowing exhaust gas reacts with the O₂ ⁻ or O²⁻ on the surface of the platinum Pt and becomes NO₂ (2NO+O₂ →2NO₂). Subsequently, a part of the produced NO₂ is oxidized on the platinum Pt and absorbed into the absorbent. While bonding with the barium oxide BaO, it is diffused in the absorbent in the form of nitric acid ions NO₃ ⁻ as shown in FIG. 5(A). In this way, NO_(x) is absorbed into the NO_(x) absorbent 17.

So long as the oxygen concentration in the inflowing exhaust gas is high, the NO₂ is produced on the surface of the platinum Pt, and so long as the NO_(x) absorption ability of the absorbent is not saturated, the NO₂ is absorbed into the absorbent and nitric acid ions NO₃ ⁻ are produced. Contrary to this, when the oxygen concentration in the inflowing exhaust gas is lowered and the production of NO₂ is lowered, the reaction proceeds in an inverse direction (NO_(x) ⁻ →NO₂), and thus nitric acid ions NO₃ ⁻ in the absorbent are released in the form of NO₂ from the absorbent. Namely, when the oxygen concentration in the inflowing exhaust gas is lowered, the NO_(x) is released from the NO_(x) absorbent 17. As shown in FIG. 4, when the degree of leanness of the inflowing exhaust gas becomes low, the oxygen concentration in the inflowing exhaust gas is lowered, and accordingly when the degree of leanness of the inflowing exhaust gas is lowered, the NO_(x) is released from the NO_(x) absorbent 17 even if the air-fuel ratio of the inflowing exhaust gas is lean.

On the other hand, at this time, when the air-fuel ratio of the air-fuel mixture fed into the combustion chamber 3 is made rich and the air-fuel ratio of the inflowing exhaust gas becomes rich, as shown in FIG. 4, a large amount of unburnt HC and CO is discharged from the engine, and these unburnt HC and CO react with the oxygen O₂ ⁻ or O²⁻ on the platinum Pt and are oxidized. Also, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the oxygen concentration in the inflowing exhaust gas is extremely lowered, and therefore the NO₂ is discharged from the absorbent. This NO₂ reacts with the unburnt HC and CO as shown in FIG. 5(B) and is reduced. In this way, when the NO₂ no longer exists on the surface of the platinum Pt, the NO₂ is successively released from the absorbent. Accordingly, when the air-fuel ratio of the inflowing exhaust gas is made rich, the NO_(x) is released from the NO_(x) absorbent 19 in a short time.

Namely, when the air-fuel ratio of the inflowing exhaust gas is made rich, first of all, the unburnt HC and CO immediately react with the O₂ ⁻ or O²⁻ on the platinum Pt and are oxidized, and subsequently if the unburnt HC and CO still remain even though the O₂ ⁻ or O²⁻ on the platinum Pt is consumed, the NO_(x) released from the absorbent and the NO_(x) discharged from the engine are reduced by these unburnt HC and CO. Accordingly, when the air-fuel ratio of the inflowing exhaust gas is made rich, the NO_(x) absorbed in the NO_(x) absorbent 17 is released in a short time and in addition this released NO_(x) is reduced, and therefore the discharge of NO_(x) into the atmosphere can be blocked. Also, since the NO_(x) absorbent 17 has the function of a reduction catalyst, even if the air-fuel ratio of the inflowing exhaust gas is made the stoichiometric air-fuel ratio, the NO_(x) released from the NO_(x) absorbent 17 can be reduced. However, where the air-fuel ratio of the inflowing exhaust gas is made the stoichiometric air-fuel ratio, the NO_(x) is released merely gradually from the NO_(x) absorbent 17, and therefore a slightly long time is required for releasing all NO_(x) absorbed in the NO_(x) absorbent 17.

When the degree of leanness of the inflowing exhaust gas is lowered as mentioned before, even if the air-fuel ratio of the inflowing exhaust gas is lean, the NO_(x) is released from the NO_(x) absorbent 17. Accordingly, so as to release the NO_(x) from the NO_(x) absorbent 17, it is satisfactory if the concentration of oxygen in the inflowing exhaust gas is lowered. Note, even if the NO_(x) is released from the NO_(x) absorbent 17, when the air-fuel ratio of the inflowing exhaust gas is lean, the NO_(x) is not reduced in the NO_(x) absorbent 17, and accordingly, in this case, it is necessary to provide a catalyst which can reduce the NO_(x) downstream of the NO_(x) absorbent 17 or supply a reducing agent downstream of the NO_(x) absorbent 17. Of course, it is also possible to reduce the NO_(x) downstream of the NO_(x) absorbent 17 in this way, but it is rather preferable that the NO_(x) be reduced in the NO_(x) absorbent 17. Accordingly, in the embodiment according to the present invention, when the NO_(x) should be released from the NO_(x) absorbent 17, the air-fuel ratio of the inflowing exhaust gas is made rich, whereby the NO_(x) released from the NO_(x) absorbent 17 is reduced in the NO_(x) absorbent 17.

However, in the embodiment according to the present invention, as mentioned above, during full load operation, the air-fuel mixture fed into the engine cylinder 3 is made rich and during high load operation, the air-fuel mixture is made the stoichiometric air-fuel ratio, so during the full load operation and the high load operation, the NO_(x) is released from the NO_(x) absorbent 17. However, if the frequency of this full load operation or high load operation is small, then even if the NO_(x) is released from the NO_(x) absorbent 17 only during full load operation and high load operation, the absorption capacity of the NO_(x) by the NO_(x) absorbent 17 will end up becoming saturated during the time when a lean air-fuel mixture is burnt and therefore it will end up becoming impossible for the NO_(x) absorbent 17 to absorb the NO_(x). Accordingly, in the embodiment according to the present invention, when a lean air-fuel mixture continues to be burnt, the air-fuel mixture fed into the combustion chamber 3 is cyclically made rich and during this time the NO_(x) is released from the NO_(x) absorbent 17.

In this case, however, if the cycle at which the air-fuel mixture fed into the engine cylinder 3 is made rich is long, then the NO_(x) absorbing capacity of the NO_(x) absorbent 17 will end up becoming saturated during the time the lean air-fuel mixture is being burnt and therefore the NO_(x) can no longer be absorbed in the NO_(x) absorbent 17, so there will be the problem that NO_(x) will end up being released into the atmosphere. As opposed to this, even if an engine operating state where a large amount of NO_(x) is discharged from the engine continues, if the cycle at which the air-fuel mixture is made lean is shortened so that the NO_(x) is released from the NO_(x) absorbent 17 before the NO_(x) absorbing capacity of the NO_(x) absorbent 17 becomes saturated, then this time the problem will arise of an increase of the amount of fuel consumption.

Therefore, in the present invention, the amount of NO_(x) which is absorbed in the NO_(x) absorbent 17 is found and the air-fuel mixture is made rich when the amount of the NO_(x) absorbed in the NO_(x) absorbent 17 exceeds a predetermined allowable value. If the air-fuel mixture is made rich when the amount of the NO_(x) absorbed in the NO_(x) absorbent 17 exceeds a predetermined allowable value, then the NO_(x) absorbing capacity of the NO_(x) absorbent 17 will never become saturated, so the NO_(x) will no longer be released into the atmosphere and, further, the frequency at which the air-fuel mixture is made rich can be reduced as well, so it is possible to suppress an increase in the amount of the fuel consumption.

However, when finding the amount of NO_(x) being absorbed in the NO_(x) absorbent 17, it is difficult to directly find the amount of NO_(x) being absorbed in the NO_(x) absorbent 17. Therefore, in the present invention, the amount of the NO_(x) absorbed in the NO_(x) absorbent 17 is estimated from the amount of NO_(x) in the exhaust gas discharged from the engine. That is, the higher the rotational speed N of the engine, the larger the amount of exhaust gas discharged per unit time from the engine, so as the engine rotational speed N becomes higher, the amount of NO_(x) discharged from the engine per unit time increases. Further, the higher the engine load, that is, the higher the absolute pressure PM in the surge tank 10, the greater the amount of the exhaust gas discharged from the combustion chambers 3 and further the higher the combustion temperature, so the higher the engine load, that is, the higher the absolute pressure PM in the surge tank 10, the greater the amount of NO_(x) discharged from the engine per unit time.

FIG. 6(A) shows the relationship between the amount of the NO_(x) discharged from the engine per unit time, the absolute pressure PM in the surge tank 10, and the engine rotational speed N as found by experiments. In FIG. 6(A), the curves show the identical amounts of NO_(x). As shown in FIG. 6(A), the amount of NO_(x) discharged from the engine per unit time becomes larger the higher the absolute pressure PM in the surge tank 10 and becomes larger the higher the engine rotational speed N. Note that the amount of NO_(x) shown in FIG. 6(A) is stored in the ROM 32 in advance in the form of a map as shown in FIG. 6(B).

On the other hand, FIG. 7 shows the relationship between the absorption capacity NO_(x) CAP which can be absorbed by the NO_(x) absorbent 17 and the temperature T of the exhaust gas, which represents the temperature of the NO_(x) absorbent 17. If the temperature of the NO_(x) absorbent 17 becomes lower, that is, the temperature T of the exhaust gas becomes lower, the oxidation action of the NO_(x) (2NO+O₂ →2NO₂) is weakened, and therefore the NO_(x) absorption capacity NO_(x) CAP is lowered. Further, if the temperature of the NO_(x) absorbent 17 becomes higher, that is, the temperature T of the exhaust gas becomes higher, the NO_(x) absorbed in the NO_(x) absorbent 17 is decomposed and naturally released, so the NO_(x) absorption capacity NO_(x) CAP is lowered. Accordingly, the NO_(x) absorption capacity NO_(x) CAP becomes larger between about 300° C. to about 500° C.

On the other hand, FIG. 8 shows the results of experiments on the amount of NO_(x) released from the NO_(x) absorbent 17 when switching the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent 17 from lean to rich. Note that the solid line in FIG. 8 shows the state when the temperature of the NO_(x) absorbent 17, that is, the temperature T of the exhaust gas, is high, while the broken line shows when the temperature of the NO_(x) absorbent 17, that is, the temperature T of the exhaust gas, is low. The rate of decomposition of the NO_(x) in the NO_(x) absorbent 17 becomes faster the higher the temperature of the NO_(x) absorbent 17. Therefore, when the temperature of the NO_(x) absorbent 17 is high, as shown by the solid line in FIG. 8, that is, when the temperature T of the exhaust gas is high, a large amount of NO_(x) is released from the NO_(x) absorbent 17 in a short time, while when the temperature of the NO_(x) absorbent 17, that is, the temperature T of the exhaust gas, is low, as shown by the broken line in FIG. 8, a small amount of NO_(x) is continually released from the NO_(x) absorbent 17 over a long period. That is, the higher the temperature T of the exhaust gas, the greater the amount of NO_(x) released from the NO_(x) absorbent 17 per unit time and the shorter the release time of the NO_(x).

When the amount of the unburnt HC and CO discharged from the engine, however, is smaller than the amount which can reduce the total NO_(x) released from the NO_(x) absorbent 17, part of the NO_(x) is released into the atmosphere without being reduced, while when the amount of unburnt HC and CO discharged from the engine is greater than the amount able to reduce the total NO_(x) released from the NO_(x) absorbent 17, the excess unburnt HC and CO are released into the atmosphere. Accordingly, to prevent the NO_(x) and the unburnt HC and CO from being released into the atmosphere, it is necessary to discharge exactly the amount of the unburnt HC and CO from the engine needed to reduce the NO_(x) released from the NO_(x) absorbent 17. Toward this end, it becomes necessary to increase the amount of the unburnt HC and CO in accordance with the curve shown in FIG. 8.

As mentioned earlier, however, the amount of the unburnt HC and CO discharged from the engine is proportional to the degree of richness of the air-fuel mixture fed into the combustion chamber 3. Therefore, in the embodiment according to the present invention, as shown in FIG. 9, the value of the correction coefficient k with respect to the basic fuel injection time TP, that is, the degree of richness of the air-fuel mixture, is made to change in accordance with a pattern as close as possible to the pattern of change of the concentration of NO_(x) shown in FIG. 8. Note that here, the correction coefficient k has the relationship K=1+k with the above-mentioned correction coefficient K and therefore when k=0, the air-fuel mixture becomes the stoichiometric air-fuel ratio while when k>0, the air-fuel mixture becomes rich.

As shown by the solid line in FIG. 9, when the NO_(x) is to be released from the NO_(x) absorbent 17, the correction coefficient k is made to rise by α increments with each passing of the unit time until the time C reaches C₁. Next, when the time C is between C₁ and C₂, the correction coefficient k is held constant, then when the time C exceeds C₂, the correction coefficient k is made to descend in β decrements with each unit time. The values of these α, β, C₁ and C₂ are set so that the pattern of change of the correction coefficient k becomes as close as possible to the pattern of change of the concentration of NO_(x) shown by the solid line in FIG. 8.

On the other hand, the pattern of change of the correction coefficient k when the temperature of the NO_(x) absorbent 17, that is, the temperature T of the exhaust gas, is low, is also set so that it becomes as close as possible to the pattern of change of the concentration of NO_(x) when the temperature T of the exhaust gas is low, as shown by the broken line in FIG. 8. In this case, to make the pattern of change of the correction coefficient k in FIG. 9 like the broken line, it is understood that it is sufficient to make both α and β smaller and make C₁ and C₂ larger. That is, to make the pattern of change of the correction coefficient k close to the pattern of change of the concentration of NO_(x) shown in FIG. 8, it is sufficient to make α and β larger and make C₁ and C₂ smaller as the temperature T of the exhaust gas becomes higher, as shown in FIG. 10. Note that the relationship between C₁, C₂ , α, and β and the temperature T of the exhaust gas shown in FIG. 10 is stored in advance in the ROM 32.

Note that in the embodiment according to the present invention, provision is made of a temperature sensor 25 for detecting the temperature T of the exhaust gas and accordingly the NO_(x) absorption capacity NO_(x) CAP shown in FIG. 7 and the α, β, C₁, and C₂ shown in FIG. 10 are determined based on the temperature T of the exhaust gas detected by this temperature sensor 25. The temperature T of the exhaust gas, however, can be estimated from the absolute pressure PM in the surge tank 10 and the engine rotational speed N. Therefore, instead of providing the temperature sensor 25, it is possible to store the temperature T of the exhaust gas in the ROM 32 in advance in the form of a map as shown in FIG. 11 and determine the NO_(x) absorption capacity NO_(x) CAP and α, β, C₁, and C₂ based on the temperature T of the exhaust gas obtained from this map.

Next, an explanation will be made of the first embodiment of control of the release of NO_(x) with reference to FIG. 12 to FIG. 14.

FIG. 12 and FIG. 13 show a time interruption routine executed by interruption every predetermined time.

Referring to FIG. 12 and FIG. 13, first, at step 100, it is judged if a NO_(x) release flag showing that the NO_(x) should be released from the NO_(x) absorbent 17 is set or not. When the NO_(x) release flag is not set, the routine proceeds to step 101, where it is judged if the correction coefficient K is smaller than 1.0, that is, if the operating state is one in which the air-fuel mixture should be made lean. When K<1.0, that is, when the operating state is one in which the air-fuel mixture should be made lean, the routine proceeds to step k₂ where the count D is made zero, then the routine proceeds to step 103.

At step 103, the NO_(x) amount Nij discharged from the engine per unit time is calculated from the map shown in FIG. 6(B) based on the absolute pressure PM in the surge tank 10, detected by the pressure sensor 19, and the engine rotational speed N. Next, at step 104, the NO_(x) amount Nij is multiplied by the interruption time interval Δt and the product Nij·ΔT is added to ΣNO_(x). The product Nij·Δt shows the amount of the NO_(x) discharged from the engine during the interruption time interval Δt. At this time, the NO_(x) discharged from the engine is absorbed by the NO_(x) absorbent 17, so ΣNO_(x) shows the estimated value of the amount of NO_(x) absorbed in the NO_(x) absorbent 17.

Next, at step 105, the NO_(x) absorption capacity NO_(x) CAP is calculated from the relationship shown in FIG. 7 based on the temperature T of the exhaust gas detected by the temperature sensor 25. Next, at step 106, it is judged if the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 has exceeded the NO_(x) absorption capacity NO_(x) CAP. When ΣNO_(x) ≦NO_(x) CAP, the processing cycle is completed. At this time, a lean air-fuel mixture is burned and the NO_(x) discharged from the engine is absorbed in the NO_(x) absorbent 17.

On the other hand, if it is judged at step 106 that ΣNO_(x) >NO_(x) CAP, that is, the NO_(x) absorption capacity of the NO_(x) absorbent 17 is saturated, the routine proceeds to step 107, where the NO_(x) release flag is set. Next, at step 108, C₁, C₂, α, and β are calculated from the relation shown in FIG. 10 based on the temperature T of the exhaust gas and the processing cycle is ended. If the NO_(x) release flag is set, at the next processing cycle, the routine proceeds from step 100 to step 109, where the count C is incremented by one. Next, at step 110, it is judged if the count C is smaller than C₁. When C<C₁, the routine proceeds to step 111, where α is added to the correction coefficient k. Next, the processing cycle is ended. The action of addition of α to the correction coefficient k is performed continuously until C≧C₁. Accordingly, the value of the correction coefficient k during this time continues to increase as shown in FIG. 9.

On the other hand, if it is judged at step 110 that C≧C₁, the routine proceeds to step 112, where it is judged if the count C has become smaller than C₂. When C<C₂, the processing cycle is ended. Therefore, as shown in FIG. 9, the correction coefficient k is held constant until C≧C₂.

Next, at step 112, when it is judged that C≧C₂, the routine proceeds to step 113, where β is subtracted from the correction coefficient k. Next, at step 113, it is judged if the correction coefficient k has become zero or a negative number. When k>0, the processing cycle is ended. Accordingly, as shown in FIG. 9, the correction coefficient k is reduced until k≦0. Note that, as mentioned later, if k>0, the air-fuel mixture fed to the combustion chamber 3 is made rich and during this time the degree of richness is changed by the pattern shown in FIG. 9.

On the other hand, if it is judged at step 114 that K≦0, the routine proceeds to step 115, where the NO_(x) release flag is reset. Next, at step 116, ΣNO_(x) is made zero. That is, at this time, it is considered that all of the NO_(x) which had been absorbed in the NO_(x) absorbent 17 is released, so the estimated value ΣNO_(x) of the NO_(x) absorbed in the NO_(x) absorbent 17 is made zero. Next, at step 117, the count C and the correction coefficient k are made zero and the processing cycle is ended.

On the other hand, if it is judged at step 101 that k≧1.0, that is, when the engine operating state is one in which the air-fuel mixture should be made rich or the stoichiometric air-fuel ratio, the routine proceeds to step 118, where the count D is incremented by one. Next, at step 119, it is judged if the count D has become larger than the constant value D₀. When β>D₀, the routine proceeds to step 120, where ΣNO_(x) is made zero. That is, when the combustion of the rich air-fuel ratio or the stoichiometric air-fuel ratio continues for a certain time, it may be considered that all of the NO_(x) has been released from the NO_(x) absorbent 17, so at this time the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is made zero.

FIG. 14 shows the routine for calculation of the fuel injection time TAU. This routine is repeatedly executed.

Referring to FIG. 14, first, at step 150, a basic fuel injection time TP is calculated from a map indicated in FIG. 2. Subsequently, at step 151, the correction coefficient K shown in FIG. 3, which is determined in accordance with the operating state of the engine, is calculated. Next, at step 152, it is judged if the NO_(x) release flag is set or not. When the NO_(x) release flag is not set, the routine proceeds to step 153, where the correction coefficient K is made K_(t). Next, at step 155, K_(t) is multiplied with the basic fuel injection time TP, whereby the fuel injection time TAU is calculated. Accordingly, at this time, the air-fuel mixture which is fed into the combustion chamber 3 is made lean, the stoichiometric air-fuel ratio, or rich in accordance with the operating state of the engine as shown in FIG. 3.

On the other hand, when it is judged at step 152 that the NO_(x) release flag is set, the routine proceeds to step 154, where it is made the sum (k+1) of correction coefficient k calculated by the routine shown in FIG. 12 and FIG. 13 and 1, then the routine proceeds to step 155. Next, at this time, the air-fuel mixture fed to the combustion chamber 3 is made rich, then the degree of richness is changed by the pattern shown in FIG. 9.

FIG. 15 to FIG. 20 show a second embodiment. As mentioned earlier, in the first embodiment, when the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 exceeded the NO_(x) absorption capacity NO_(x) CAP of the NO_(x) absorbent 17, the air-fuel ratio of the air-fuel mixture fed to the combustion chamber 3 was switched from lean to rich and the NO_(x) was released from the NO_(x) absorbent 17, but further, even during engine high load operation and engine full load operation, a releasing action of the NO_(x) from the NO_(x) absorbent 17 is performed. In the second embodiment, even in engine operating states other than engine high load operation and engine full load operation, the air-fuel mixture is made rich and a NO_(x) release action from the NO_(x) absorbent 17 is performed. At this time, when all of the NO_(x) absorbed in the NO_(x) absorbent 17 is considered to have been released, the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is made zero.

That is, in the first embodiment, as will be understood from FIG. 3, when an acceleration operation is performed in which the engine load is made to increase in the low load region, that is, when a gradual acceleration operation is performed, the air-fuel mixture is maintained lean, while when an acceleration operation is performed in which the engine load shifts from the low load to high load, that is, a rapid acceleration operation is performed, the air-fuel mixture is switched from lean to the stoichiometric air-fuel ratio. As opposed to this, in the second embodiment, during rapid acceleration operation, the air-fuel mixture is made a rich air-fuel mixture in accordance with the degree of acceleration to secure a good acceleration operation and at that time the NO_(x) release action is performed.

Further, when a shift down is performed in the automatic transmission 22, the engine rotational speed increases, but at this time, if there is a delay in the increase of the engine rotational speed, a torque shock is produced when the shift down occurs. Therefore, in the second embodiment, when a shift down is performed, the air-fuel mixture is made rich to immediately raise the engine rotational speed and thereby the occurrence of a torque shock is inhibited. At this time too, the NO_(x) release action is performed.

Further, in the first embodiment, when the estimated value ΣNO_(x) of the NO_(x) absorbed in the NO_(x) absorbent 17 reaches the NO_(x) absorption capacity NO_(x) CAP shown in FIG. 7, the air-fuel mixture is switched from lean to rich and a NO_(x) release action is performed, but in the second embodiment, when the estimated value ΣNO_(x) of the NO_(x) absorbed in the NO_(x) absorbent 17 reaches 70 percent of the NO_(x) absorption capacity NO_(x) CAP shown in FIG. 7, the air-fuel mixture is switched from lean to rich and the NO_(x) release action is performed.

Further, in the second embodiment, during idling operation in which the throttle valve 14 is open for idling and during a deceleration operation, the air-fuel ratio of the air-fuel mixture fed to the combustion chamber 3 is made the stoichiometric air-fuel ratio. However, when the air-fuel mixture is made rich to cause the NO_(x) to be released, it takes less in fuel consumption and there is less fluctuation in the torque if the air-fuel ratio of the air-fuel mixture is switched between the stoichiometric air-fuel ratio and rich rather than if the air-fuel ratio of the air-fuel mixture is switched between lean and rich. Accordingly, in the second embodiment, to increase the opportunities for causing the NO_(x) to be released by switching the air-fuel ratio of the air-fuel mixture from the stoichiometric air-fuel ratio to rich, in the case where the estimated value ΣNO_(x) of the amount of the NO_(x) absorbed in the NO_(x) absorbent 17 exceeds 30 percent of the NOx absorption capacity NO_(x) CAP shown in FIG. 7, when the throttle valve 14 is open in the idling position, the air-fuel mixture is temporarily made rich to perform the NO_(x) release action. In this case, if the NO_(x) release action is ended, the air-fuel mixture is maintained at the stoichiometric air-fuel ratio.

Next, an explanation will be made of a second embodiment of the control of release of NO_(x) with reference to FIG. 15 to FIG. 19.

FIG. 15 and FIG. 16 show a time interruption routine executed by interruption every predetermined time.

Referring to FIG. 15 and FIG. 16, first, at step 200, it is judged if a NO_(x) release flag showing that the NO_(x) should be released from the NO_(x) absorbent 17 is set or not. When the NO_(x) release flag is not set, the routine proceeds to step 201, where it is judged if the correction coefficient K is smaller than 1.0, that is, if the operating state is one in which the air-fuel mixture should be made lean. When K<1.0, that is, when the operating state is one in which the air-fuel mixture should be made lean, the routine proceeds to step 202.

At step 202, the NO_(x) amount Nij discharged from the engine per unit time is calculated from the map shown in FIG. 6(B) based on the absolute pressure PM in the surge tank 10, detected by the pressure sensor 19, and the engine rotational speed N. Next, at step 203, the NO_(x) amount Nij is multiplied by the interruption time interval Δt and the product Nij·ΔT is added to ΣNO_(x). The product Nij·Δt shows the estimated value of the amount of the NO_(x) absorbed in the NO_(x) absorbent 17. Next, at step 204, it is judged if the estimated value ΣNO_(x) of the amount of the NO_(x) absorbed in the NO_(x) absorbent 17 is greater than 30 percent CAP, that is, 30 percent of the NO_(x) absorption capacity NO_(x) CAP shown in FIG. 7. When ΣNO_(x) ≦30 percent CAP, the routine proceeds to step 205, where the enable flag is reset, then the processing cycle is ended. As opposed to this, when ΣNO_(x) >30 percent CAP, the routine proceeds to step 206, where the enable flag is set, then the routine proceeds to step 207.

At step 207, it is judged if the estimated value ΣNO_(x) of the amount of the NO_(x) absorbed in the NO_(x) absorbent 17 is greater than 70 percent CAP, that is, 70 percent of the NO_(x) absorption capacity NO_(x) CAP shown in FIG. 7. When ΣNO_(x) ≦70 percent CAP, the processing cycle ends.

On the other hand, if it is judged at step 207 that ΣNO_(x) >70 percent CAP, that is, if it is judged that over 70 percent of the NO_(x) of the NO_(x) absorption capacity is absorbed in the NO_(x) absorbent 17, the routine proceeds to step 208, where the NO_(x) release flag is set. Next, at step 209, C₁, C₂, α, and β are calculated from the relationship shown in FIG. 10 on the basis of the temperature T of the exhaust gas and the processing cycle ends. If the NO_(x) release flag is set, at the next processing cycle, the routine proceeds from step 200 to step 210, where the count is incremented by one. Next, at step 111, it is judged if the count C is smaller than C₁. When C<C₁, the routine proceeds to step 212, α is added to the correction coefficient k. Next, the processing cycle ends. The action of addition of α to the correction coefficient k is performed continuously until C≧C₁ . Accordingly, the value of the correction coefficient k continues to increase during this period as shown in FIG. 9.

On the other hand, if it is judged at step 211 that C≧C₁, the routine proceeds to step 213, where it is judged if the count C has become smaller than even C₂. When C<C₂, the processing cycle ends. Therefore, as shown in FIG. 9, the correction coefficient k is held constant until C≧C₂.

Next, if it is judged at step 213 that C≧C₂, the routine proceeds to step 214, where β is subtracted from the correction coefficient k. Next, at step 215, it is judged if the correction coefficient k has become zero or a negative number. When k>0, the processing cycle ends. Therefore, as shown in FIG. 9, the correction coefficient k is reduced until k≦0. Note that as mentioned later, when k>0, the air-fuel mixture fed into the combustion chamber 3 is made rich and during this time the degree of richness is changed by the pattern shown in FIG. 9.

On the other hand, if it is judged at step 215 that k≦0, the routine proceeds to step 216, where the NO_(x) release flag is reset. Next, at step 217, ΣNO_(x) is made zero. That is, at this time, it is considered that all of the NO_(x) which had been absorbed in the NO_(x) absorbent 17 has been released, so the estimated valve ΣNO_(x) of the NO_(x) absorbed in the NO_(x) absorbent 17 is made zero. Next, at step 218, the count C and the correction coefficient k are made zero and the processing cycle is ended.

On the other hand, if it is judged at step 201 that k≧1.0, that is, the engine operating state is one in which the air-fuel mixture should be made rich or the stoichiometric air-fuel ratio, the routine proceeds to step 219, where it is judged if the vehicle speed V is greater than a constant value, for example, 130 km/h, from the output signal of the vehicle speed sensor 24. If the operation is performed so that the vehicle speed V exceeds 130 km/h, the NO_(x) is completely released from the NO_(x) absorbent 17, so at this time the routine proceeds to step 221, where the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is made zero. As opposed to this, when V≦130 km/h, the routine proceeds to step 220, where it is judged if a predetermined time has past from when k≧1.0. When a predetermined time has past, the routine proceeds to step 221, where ΣNO_(x) is made zero. That is, when the combustion of a rich air-fuel mixture or an air-fuel mixture with a stoichiometric air-fuel ratio continues for a predetermined time, it is considered that all the NO_(x) has been released from the NO_(x) absorbent 17, so at this time, the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is made zero.

FIG. 17 to FIG. 19 show the routine for calculating the fuel injection time TAU. This routine is repeatedly executed.

Referring to FIG. 17 to FIG. 19, first, at step 250, the basic fuel injection time TP is calculated from the map shown in FIG. 2. Next, at step 251, the correction coefficient K shown in FIG. 3, determined in accordance with the operating state of the engine, is calculated. Next, at step 252, it is judged if the throttle valve 14 is open for idling on the basis of the output signal of the idle switch 20. When the throttle valve 14 is not open for idling, the routine proceeds to step 261, where the rich flag is reset, then the routine proceeds to step 262.

At step 262, it is judged if the correction coefficient K is smaller than 1.0. When K≧1.0, that is, when the air-fuel ratio of the air-fuel mixture should be made rich or the stoichiometric air-fuel ratio, the routine jumps to step 268. As opposed to this, when K≦1.0, that is, when the air-fuel ratio of the air-fuel mixture should be made lean, the routine proceeds to step 263, where the pressure difference ΔPM between the current absolute pressure PM in the surge tank 10 and the absolute pressure PM₁ in the surge tank 10 which is detected in the previous processing cycle is calculated. Next, at step 264, it is judged if the pressure difference ΔPM is larger than a constant value X₀, that is, if a rapid acceleration operation is underway. When ΔPM≦X₀, that is, when a rapid acceleration operation is not underway, the routine proceeds to step 268.

At step 268, it is judged if a shift down action of the automatic transmission is being performed on the basis of the output signal of the gear position detector 23. When a shift down action is not underway, the routine jumps to step 272. At step 272, it is judged if the NO_(x) release flag is set or not. When the NO_(x) release flag is not set, the routine proceeds to step 273, where the correction coefficient K is made K_(t). Next, at step 275, K_(t) is multiplied with the basic fuel injection time TP, whereby the fuel injection time TAU is calculated. Accordingly, at this time, the air-fuel mixture fed into the combustion chamber 3 is made lean or the stoichiometric air-fuel ratio or rich in accordance with the operating state of the engine as shown in FIG. 3.

On the other hand, if it is judged at step 274 that the NO_(x) release flag is set, the routine proceeds to step 274, where K_(t) is made the sum of the correction coefficient k calculated by the routine shown in FIG. 15 and FIG. 16 and 1, then the routine proceeds to step 275. Therefore, at this time, the air-fuel mixture fed into the combustion chamber 3 is made rich. At this time, the degree of richness is changed by the pattern shown in FIG. 9.

On the other hand, when it is judged at step 252 that the throttle valve 14 is open in the idling position, the routine proceeds to step 253, where it is judged if the rich flag is set or not. If the rich flag is not set, the routine proceeds to step 254, where it is judged if the enable flag is set or not. When the enable flag is set, the routine proceeds to step 255, where the rich flag is set, then at step 256, the correction coefficient K_(t) is made 1.2. Next, the routine proceeds to step 275. As opposed to this, when the enable flag is not set, the routine proceeds to step 260, where the correction coefficient K_(t) is made 1.0, then the routine proceeds to step 275.

Accordingly, when the throttle valve 14 is open to the idling position, if the enable flag is set, that is, when over 30 percent of the NO_(x) absorption capacity NO_(x) CAP of the NO_(x) is absorbed in the NO_(x) absorbent 17, the air-fuel mixture is made rich. At this time, when the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is less than 30 percent of the NO_(x) absorption capacity NO_(x) CAP, the air-fuel ratio of the air-fuel mixture is made the stoichiometric air-fuel ratio.

If the rich flag is set, the routine proceeds from step 253 to step 257, where it is judged if a predetermined time has elapsed since the rich flag has been set. When the predetermined time has elapsed, the routine proceeds to step 258, where the rich flag is reset, then at step 259 the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is made zero. Next, at step 260, the correction coefficient K_(t) is made 1.0. If the ΣNO_(x) is made zero at step 259, the enable flag is reset in the routine shown in FIG. 15 and FIG. 16, so at the next processing cycle, the routine proceeds through steps 253 and 254 to step 260. Therefore, when the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is over 30 percent of the NO_(x) absorption capacity NO_(x) CAP, the air-fuel mixture is temporarily made rich, then the air-fuel ratio of the air-fuel mixture is made the stoichiometric air-fuel ratio.

On the other hand, if it is judged at step 264 that ΔPM >X₀, that is, during a rapid acceleration operation, the routine proceeds to step 265, where the correction coefficient K_(t) is calculated from the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 on the basis of the relationship shown in FIG. 20. As shown in FIG. 20, the correction coefficient K_(t) is larger than 1 and the correction coefficient K_(t) becomes larger the larger the estimated value ΣNO_(x). Accordingly, if a rapid acceleration operation is performed, the air-fuel mixture is made rich. Next, at step 266, it is judged if a predetermined time has elapsed since ΔPM >X₀. If the predetermined time has elapsed, the routine proceeds to step 267, where the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is made zero.

On the other hand, when it is judged at step 268 that a shift down action is underway at the automatic transmission 22, the routine proceeds to step 269, where the correction coefficient K_(t) is made 1.2. Therefore, when a shift down action is performed, it is understood, the air-fuel mixture immediately is made rich. Next, at step 270, it is judged if a predetermined time has elapsed from when the shift down action was started. When a predetermined time has elapsed, the routine proceeds to step 271, where the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is made zero.

Note that if the air-fuel mixture is made rich for a certain time to release the NO_(x), then the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is made zero at steps 259, 267, and 271, but at this time it is also possible not to make the estimated value ΣNO_(x) zero, but to make the estimated value ΣNO_(x) about 30 percent or lower of the NO_(x) absorption capacity NO_(x) CAP.

FIG. 21 shows another embodiment of the internal combustion engine. In this embodiment, an outlet side of a casing 18 is connected through an exhaust pipe 27 to a catalytic converter 29 housing a three-way catalyst. This three-way catalyst 28, as is well known, exhibits a high purification efficiency with respect to CO, HC, and NO_(x) when the air-fuel ratio is maintained near the stoichiometric air-fuel ratio, but the three-way catalyst 28 also has a high purification efficiency with respect to NO_(x) even when the air-fuel ratio becomes rich to a certain extent. In the embodiment shown in FIG. 21, a three-way catalyst 28 is provided downstream of the NO_(x) absorbent 17 so as to remove the NO_(x) using this characteristic.

That is, as is mentioned above, if the air-fuel mixture fed into the engine cylinder is made rich to release the NO_(x) from the NO_(x) absorbent 17, the NO_(x) absorbed in the NO_(x) absorbent 17 is rapidly released from the NO_(x) absorbent 17. At this time, the NO_(x) is reduced during its release, but there is a possibility that all of the NO_(x) will not be reduced. If the three-way catalyst 28 is disposed downstream of the NO_(x) absorbent 17, however, the NO_(x) which could not be reduced at the time of the release of the NO_(x) is reduced by the three-way catalyst 28. Accordingly, by disposing the three-way catalyst 28 downstream of the NO_(x) absorbent 17, it becomes possible to improve considerably the purification performance of the NO_(x).

In the embodiments discussed up to here, use was made, as the NO_(x) absorbent, of a NO_(x) absorbent 17 comprised of at least one of an alkali metal, alkali earth, and rare earth and a precious metal carried on alumina. Instead of using such a NO_(x) absorbent 17, however, it is also possible to use a complex oxide of an alkali earth and copper, that is, a NO_(x) absorbent of the Ba-Cu-O system. As such a complex oxide of an alkali earth and copper, use may be made for example of MnO₂ ·BaCuO₂. In this case, it is also possible to add platinum Pt or cerium Ce. In a NO_(x) absorbent of the MnO₂ ·BaCuO₂ system, the copper Cu performs the same catalytic function as the platinum Pt in the NO_(x) absorbent 17 spoken of up to now. When the air-fuel ratio is lean, the NO_(x) is oxidized by the copper (2NO+O.sub. 2 →2NO₂) and dispersed in the absorbent in the form of nitric acid ions NO₃ ⁻.

On the other hand, if the air-fuel ratio is rich, similarly, NO_(x) is released from the absorbent. This NO_(x) is reduced by the catalytic action of the copper Cu. The NO_(x) reducing ability of copper Cu, however, is weaker than the NO_(x) reducing ability of platinum Pt and therefore when using an absorbent of the Ba-Cu-O system, the amount of NO_(x) which is not reduced at the time of release of the NO_(x) becomes somewhat greater than with the NO_(x) absorbent 17 discussed up to now. Therefore, when using an absorbent of the Ba-Cu-O system, as shown in FIG. 21, it is preferable to dispose a three-way catalyst 28 downstream of the absorbent.

FIG. 22 and FIG. 27 show the case of application of the present invention to a diesel engine. Note that in FIG. 22 and FIG. 27, constituent elements the same as those in FIG. 1 are given the same reference numerals.

In a diesel engine, usually, during all operating states, combustion is performed with an air excess rate of over 1.0, that is, with the average air-fuel ratio of the air-fuel mixture in the combustion chamber 3 in a lean state. Accordingly, at this time, the NO_(x) which is discharged is absorbed in the NO_(x) absorbent 17. On the other hand, when NO_(x) is to be released from the NO_(x) absorbent 17, hydrocarbons are fed into the engine exhaust passage upstream of the NO_(x) absorbent 17, whereby the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent 17 becomes rich.

Referring to FIG. 22, in this embodiment, provision is made of a load sensor 51 which generates an output voltage proportional to the amount of depression of the accelerator pedal 50. The output voltage of the load sensor 51 is input to the input port 35 through an AD converter 40. Further, in this embodiment, an intake shutoff valve 52 is disposed in the intake duct 12, which intake shutoff valve 52 is connected to a diaphragm 54 of the vacuum diaphragm apparatus 53. The diaphragm vacuum chamber 55 of the vacuum diaphragm apparatus 53 is connected selectively to the atmosphere or a vacuum tank 57 through a solenoid switching valve 56. On the other hand, the output port 36 of the electronic control unit 30 is connected through a corresponding drive circuit 39 to a solenoid switching valve 56. The diaphragm vacuum chamber 55 is usually open to the atmosphere. At this time, the intake shutoff valve 52 is held in the fully open position as shown in FIG. 22.

Further, a reducing agent feed valve 58 is disposed in the exhaust pipe 16, which reducing agent feed valve 58 is connected through a feed pump 59 to a reducing agent tank 60. The output port 36 of the electronic control unit 30 is connected through the corresponding drive circuit 39 to the reducing agent feed valve 58 and the feed pump 59. Inside the reducing agent tank 60 is filled a hydrocarbon such as gasoline, isooctane, hexane, heptane, gas oil, and kerosine or a hydrocarbon which can be stored in a liquid form, such as butane or propane.

In the diesel engine shown in FIG. 22 too, the amount of the NO_(x) which is absorbed in the NO_(x) absorbent 17 is estimated from the amount of NO_(x) in the exhaust gas discharged from the engine. That is, in the diesel engine as well, the higher the engine rotational speed N, the greater the amount of the exhaust gas discharged per unit time from the engine, so along with a rise in the engine rotational speed N, the amount of NO_(x) discharged per unit time from the engine increases. Further, the higher the engine load, that is, the greater the amount of depression of the accelerator pedal 50, the greater the amount of exhaust gas discharged from the combustion chambers 3 and further the higher the combustion temperature, so the higher the engine load, that is, the greater the amount of depression of the accelerator pedal 50, the greater the amount of the NO_(x) discharged per unit time from the engine.

FIG. 23(A) shows the relationship between the amount of NO_(x) discharged from an engine per unit time, the amount of depression Acc of the accelerator pedal 50, and the engine rotational speed N, found by experiments. In FIG. 23(A), the curves show the same amounts of NO_(x). As shown in FIG. 23(A), the amount of NO_(x) which is discharged from the engine per unit time increases along with an increase in the amount of depression Acc of the accelerator pedal 50 and increases along with an increase in the engine rotational speed N. Note that the amount of NO_(x) shown in FIG. 23(A) is stored in the ROM 32 in advance in the form of a map shown in FIG. 23(B).

In a diesel engine, the air-fuel mixture inside the combustion chamber 3 is made to burn in an excessive air state, that is, in a state with the average air-fuel ratio lean. At this time, the NO_(x) discharged from the engine is absorbed in the NO_(x) absorbent 17. FIG. 24 shows the relationship between the amount of NO_(x) absorbed in the NO_(x) absorbent 17 and the concentration of NO_(x) in the exhaust gas flowing out from the NO_(x) absorbent 17. Further, in FIG. 24, the amount of absorption A of the NO_(x) shows the allowable absorption limit amount below which the NO_(x) absorbent 17 can absorb NO_(x) well.

As will be understood from FIG. 24, when the amount NO_(x) absorption is smaller than the allowable absorption limit amount A, all of the NO_(x) in the exhaust gas is absorbed in the NO_(x) absorbent 17, so at this time the concentration of the NO_(x) in the exhaust gas flowing out from the NO_(x) absorbent 17 becomes zero. As opposed to this, if the amount of absorption of NO_(x) exceeds the allowable absorption limit amount A, the NO_(x) absorption rate gradually falls along with the increase of the amount of absorption of NO_(x), therefore the concentration of NO_(x) in the exhaust gas flowing out from the NO_(x) absorbent 17 gradually becomes higher. At this time, if the amount of NO_(x) in the exhaust gas flowing into the NO_(x) absorbent 17 is made H₁ (=1.0), then of the NO_(x), only K₁ /H₁ (=K₁) is absorbed by the NO_(x) absorbent 17.

On the other hand, in this embodiment, when a deceleration operation is performed, the NO_(x) release action is performed. That is, when a deceleration operation is performed, the diaphragm vacuum chamber 55 is connected to the vacuum tank 57 by the switching action of the switching valve 56, whereby the intake shutoff valve 52 is made to close to close to the fully closed position. If the feed pump 61 is driven at the same time, the reducing agent feed valve 58 is made to open, whereby the hydrocarbons filled in the reducing agent tank 60 are fed from the reducing agent feed valve 58 to the inside the exhaust pipe 16. The amount of the hydrocarbons fed at this time is determined so that the air-fuel ratio of the inflowing exhaust gas flowing into the NO_(x) absorbent 17 becomes rich. Accordingly, at this time, the NO_(x) is released from the NO_(x) absorbent 17.

If the intake shutoff valve 52 is made to close in this way, the amount of exhaust gas discharged from the engine falls and therefore a small amount of reducing agent is fed, whereby the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent 17 is made rich. That is, when the NO_(x) release action is performed, it is possible to close the intake shutoff valve 52 to reduce the amount of consumption of the reducing agent. Further, the only time when there is no effect on the operating state of the engine even when the intake shutoff valve 52 is closed is during deceleration operation. Further, at this time, if the intake shutoff valve 52 is made to close, the engine brake acts powerfully. Accordingly, in the embodiment according to the present invention, the intake shutoff valve 52 is closed at the time of deceleration operation and at that time the NO_(x) release action is performed.

FIG. 25 shows the residual rate of the NO_(x) which continues to remain in the NO_(x) absorbent 17 after the start of the release of the NO_(x). As shown by the solid line in FIG. 25, if the NO_(x) release action is started, the NO_(x) residual rate gradually falls. In this case, as shown by the solid line in FIG. 25, the residual amount does not fall uniformly along with the passage of time, but the residual amount of NO_(x) falls relatively fast in the early part of the NO_(x) release action and falls relatively gently at the latter part of the same. Further, when use is made of a reducing agent with a relatively low volatility, such as gas oil or kerosine, it takes time for the reducing agent to vaporize, so as shown by the broken line in FIG. 25, even if the reducing agent is fed, the NO_(x) release action will not be performed immediately and therefore time will be taken until all of the NO_(x) is released.

As will be understood from FIG. 25, if the NO_(x) release action is performed for about the time t₀, substantially all the NO_(x) is released from the NO_(x) absorbent 17. Therefore, in the embodiment according to the present invention, when a deceleration operation is started, the intake shutoff valve 52 is made to close over the time t₀ and the reducing agent is fed into the exhaust pipe 16 over the time t₀. However, if the period of the deceleration operation is short, the NO_(x) release action ends up stopped before the NO_(x) residual rate becomes zero. In such a case, if it is judged that the release action of all of the NO_(x) has ended, then when the estimated value ΣNO_(x) of the NO_(x) absorbed in the NO_(x) absorbent 17 reaches the allowable absorption limit amount A (FIG. 24), if it is attempted to perform the NO_(x) release action once again, then the absorption ability of the NO_(x) absorbent 17 will end up becoming saturated before the NO_(x) release action is performed and therefore a large amount of NO_(x) will be released into the atmosphere.

However, when the period of the deceleration operation is short and therefore the NO_(x) residual rate does not become zero, if the NO_(x) residual rate just before the start of the deceleration operation is made H₂ (=1.0), then the NO_(x) residual rate when the NO_(x) release action is ended is expressed by K₂ /H₂ (=K₂). Accordingly, in the embodiment according to the present invention, when not all of the NO_(x) has been released when releasing the NO_(x), it is estimated that the amount of NO_(x) of K₂ ·ΣNO_(x) continues to remain in the NO_(x) absorbent 17 and the next performed NO_(x) release period is made earlier.

Next, an explanation will be given of the control of the release of NO_(x) referring to FIG. 26. Note that the NO_(x) release control routine shown in FIG. 26 is executed by interruption every predetermined time period.

Referring to FIG. 26, first, at step 300, the amount Nij of NO_(x) discharged from the engine per unit time is calculated from the map shown in FIG. 23(B) based on the amount of depression of the accelerator pedal 50 and the engine rotational speed N. Next, at step 301, it is judged if the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 has exceeded the allowable absorption limit A (FIG. 24). When ΣNO_(x) ≦A, the routine proceeds to step 302, where the interruption time interval Δt is multiplied with the amount Nij of the NO_(x). The product Nij·Δt is added to ΣNO_(x). The product Nij·Δt expresses the amount of NO_(x) discharged from the engine in the period of the interruption time interval Δt and therefore ΣNO_(x) expresses the estimated value of the amount of NO_(x) absorbed in the NO_(x) absorbent 17. Next, at step 303, the execution enable flag F₁ for enabling execution of the NO_(x) release action is reset, then the routine proceeds to step 306.

On the other hand, when it is judged at step 301 that ΣNO_(x) >A, the routine proceeds to step 304, where the product K₁ ·Nij·Δt obtained by multiplying K₁ (FIG. 24) with Nij·Δt is added to ΣNO_(x). Next, the routine proceeds to step 305, where the execution enable flag is set, then the routine proceeds to step 306. At step 306, it is judged if the execution enable flag has been set. If the execution enable flag has not been set, the routine proceeds to step 314, where the count T is made zero, then the processing cycle is ended.

On the other hand, if it is judged at step 306 that the execution enable flag has been set, the routine proceeds to step 307, where it is judged if the conditions for the NO_(x) release action, that is, the conditions for regeneration of the NO_(x), stand or not. In this case, when the amount of depression of the accelerator pedal 50 is zero and the engine rotational speed N is higher than a predetermined rotational speed, that is, during deceleration operation, it is judged that the conditions for regeneration of the NO_(x) absorbent 17 stand. Note that in this case, it is possible to add to the conditions of regeneration that the temperature of the exhaust gas be at least a temperature able to make the temperature of the NO_(x) absorbent 17 the activation temperature.

When it is judged at step 307 that the regeneration conditions stand, the routine proceeds to step 308, where the NO_(x) is regenerated. That is, the intake shutoff valve 52 is made to close and the reducing agent is fed from the reducing agent feed valve 58. Next, at step 309, the interruption time interval Δt is added to the count T. Next, at step 310, it is judged if the time T elapsed since the start of regeneration has exceeded t₀ (FIG. 25). When T <t₀, the processing cycle is ended. As opposed to this, when T≧t₀, the regeneration action of the NO_(x) absorbent 17 is stopped, then the routine proceeds to step 312, where the estimated amount ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is made zero. Next, the routine proceeds through step 314, whereafter the processing cycle is ended.

On the other hand, when the regeneration conditions no longer stand, the routine proceeds to step 311, where it is judged if the time T elapsed from the start of the regeneration exceeds a predetermined time t_(A). The predetermined time T_(A) is a time somewhat shorter than t₀ as shown in FIG. 25 and expresses a time wherein it can be deemed that the NO_(x) residual rate is zero. Accordingly, when T>T_(A), the routine proceeds to step 312, where the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is made zero.

As opposed to this, when it is judged at step 311 that T≦t_(A), that is, when the period of the deceleration operation is short and NO_(x) continues to remain in the NO_(x) absorbent 17, the routine proceeds to step 313, where the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is deemed to be ΣNO_(x) multiplied by K₂ (FIG. 25). Accordingly, in the next interruption routine, at step 302, Nij ·Δt is added to ΣNO_(x).

In the embodiment shown in FIG. 27, a NO_(x) concentration sensor 62 is disposed in the exhaust passage 60 downstream of the NO_(x) absorbent 17. This NO_(x) concentration sensor 62 generates an output voltage proportional to the NO_(x) concentration in the exhaust gas discharged from the NO_(x) absorbent 17, which output voltage is input through an AD converter 41 to the input port 35. Further, the output port 36 is connected through a corresponding drive circuit 39 to a warning lamp 63.

In this embodiment too, basically, when the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 exceeds the allowable limit amount A, if the conditions for regeneration stand, the NO_(x) is regenerated. Further, in this embodiment, if it is detected that the absorbing ability of the NO_(x) absorbent 17 has fallen due to the fact that the amount of NO_(x) actually absorbed is greater than the estimated value ΣNO_(x), the NO_(x) absorbent 17 has deteriorated, or some other reason and the absorbing ability of the NO_(x) absorbent 17 falls, the regeneration action of the NO_(x) absorbent 17 is promoted by prolonging the regeneration period of the NO_(x) absorbent 17.

Next, an explanation will be made of the control for the release of NO_(x) referring to FIG. 28 to FIG. 30. Note that the NO_(x) release control routine shown from FIG. 28 to FIG. 30 is executed by interruption every predetermined time.

Referring to FIG. 28 to FIG. 30, at step 400, the amount Nij of the NO_(x) discharged from the engine per unit time is calculated from the map shown in FIG. 23(B) on the basis of the amount of depression of the accelerator pedal 50 and the engine rotational speed N. Next, at step 401, it is judged if the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 has exceeded the allowable limit amount A (FIG. 24). When ΣNO_(x) ≦A, the routine proceeds to step 402, where the interruption time interval Δt is multiplied with the amount Nij of the NO_(x). This product Nij.Δt is added to ΣNO_(x). The product Nij·Δt expresses the amount of NO_(x) discharged from the engine in the period of the interruption time interval Δt and accordingly ΣNO_(x) expresses the estimated amount of the amount of NO_(x) absorbed in the NO_(x) absorbent 17. Next, at step 403, the execution flag F₁ for enabling execution of the NO_(x) release action is reset, then the routine proceeds to step 415.

On the other hand, it is judged at step 401 that ΣNO_(x) >A, the routine proceeds to step 404, where the NO_(x) concentration NOR detected by the NO_(x) concentration sensor 62 is read. Next, at step 405, it is judged if the NO_(x) concentration NOR is larger than a predetermined value W₀. When NOR≦W₀, the NO_(x) has not deteriorated and therefore it is considered that an amount of NO_(x) as estimated is absorbed in the NO_(x) absorbent 17, so the routine proceeds to step 406, where the regeneration promotion flag F₂ showing that the regeneration of the NO_(x) absorbent 17 should be promoted is reset. Next, at step 407, the count C is made zero, then at step 408, the product K₁ ·Nij·Δt obtained by multiplying K₁ (FIG. 24) with Nij·Δt is added to ΣNO_(x). Next, the routine proceeds to step 409, where the execution enable flag is set, then the routine proceeds to step 415. At step 415, it is judged if the execution enable flag is set. If the execution enable flag is not set, the routine proceeds to step 423, where the count T is made zero, then the processing cycle is ended.

On the other hand, if it is judged at step 415 that the execution enable flag is set,the routine proceeds to step 416, where it is judged if the condition for the NO_(x) release action, that is, the conditions for regeneration of the NO_(x) absorbent 17, stand. In this case, when the amount of depression of the accelerator pedal 50 is zero and the engine rotational speed N is higher than a predetermined rotational speed, as mentioned earlier, that is, during deceleration operation, it is judged that the conditions for regeneration of the NO_(x) absorbent 17 stand.

When it is judged at step 416 that the conditions for regeneration stand, the routine proceeds to step 417, where it is judged if the regeneration promotion flag is set. When the regeneration promotion flag is not set, the routine proceeds to step 148, where the NO_(x) absorbent 17 is regenerated. That is, the intake shutoff valve 52 is made to close and the reducing agent is fed from the reducing agent feed valve 58. Next, at step 419, the interruption time interval Δt is added to the count T. Next, at step 420, it is judged if the time T elapsed from the start of the regeneration exceeds t₀ (FIG. 25). When T<t₀, the processing cycle is ended. As opposed to this, when T≧t₀, the regeneration action of the NO_(x) absorbent 17 is stopped and the routine proceeds to step 422, where the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) is made zero. Next, the routine proceeds through step 423 and the processing cycle is ended.

On the other hand, when the regeneration conditions no longer stand, the routine proceeds to step 416, where it is judged if the time T elapsed from the start of regeneration exceeds a predetermined time t_(A). This predetermined time T_(A) is a time somewhat shorter than t₀ and expresses the time during which the NO_(x) residual rate is deemed to be zero, as mentioned earlier. Therefore, when T>t_(A), the routine proceeds to step 422, where the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is made zero.

As opposed to this, when it is judged at step 421 that T≦t_(A), that is, when the period of the deceleration operation is short and the NO_(x) continues to remain in the NO_(x) absorbent 17, the routine proceeds to step 424, where the estimated value ΣNO_(x) of the amount of the NO_(x) absorbed in the NO_(x) absorbent 17 is deemed to be the product of the multiplication of K₂ (FIG. 25) with ΣNO_(x).

On the other hand, if it is judged at step 405 that NOR>W₀, that is, if it is judged that the NO_(x) concentration is high, the routine proceeds to step 410, where the regeneration promotion flag F₂ is set, then at step 411, the count C is incremented by one. Next, at step 412, it is judged if the count C has become larger than the predetermined value C₀. When C≦C₀, the routine proceeds to step 408. Next, the routine proceeds through step 409 and 415 to step 416.

Next, if it is judged at step 416 that the regeneration conditions stand, the routine proceeds through step 417 to step 405, where the processing for promotion of regeneration is performed. In this processing of promotion of regeneration, for example, the voltage applied to the feed pump 59 is made to increase and the amount of feed of the reducing agent is made to increase. Alternatively, a burner (not shown) is disposed in the exhaust passage upstream of the NO_(x) absorbent 17 and the temperature of the exhaust gas is raised by the burner. Next, at step 426, the interruption time interval Δt is added to the count T, then at step 427, it is judged if the time T elapsed from the start of the regeneration promotion processing has become greater than a predetermined time t₁. This predetermined time t₁ is a time longer than t₀ at step 420. When T≧t₁, the regeneration promotion processing of the NO_(x) absorbent 17 is stopped, then the routine proceeds to step 428, where the estimated value ΣNO_(x) of the amount of NO_(x) absorbed in the NO_(x) absorbent 17 is made zero.

So long as the NO_(x) absorbent 17 does not deteriorate and no abnormalities occur, if the regeneration promotion processing is performed once or the regeneration promotion processing is performed several times, when ΣNO_(x) ≧A once again, it is judged at step 405 that NOR≦W₀. If the NO_(x) absorbent 17 deteriorates or an abnormality occurs, however, even if the regeneration processing is performed several times, it is judged at step 405 that NOR>W₀, therefore at step 412 it is judged that C>C₀. In this case, the routine proceeds to step 413, where for example the regeneration execution flag F₁ is reset and wasteful regeneration processing is prohibited or other abnormality processing performed. Next, at step 414, for example, the warning lamp 63 is lit and the fact that an abnormality has occurred in the NO_(x) absorbent 17 is notified to the driver. 

We claim:
 1. An exhaust purification device of an internal combustion engine which has in an engine exhaust passage a NO_(x) absorbent which absorbs NO_(x) when the air-fuel ratio of the inflowing exhaust gas is lean and which releases the absorbed NO_(x) when the oxygen concentration in the inflowing exhaust gas is reduced and which is provided with a NO_(x) estimating means for estimating the amount of the NO_(x) absorbed by the NO_(x) absorbent and a NO_(x) releasing means for reducing the oxygen concentration in the exhaust gas flowing into the NO_(x) absorbent and releasing NO_(x) from the NO_(x) absorbent when the amount of the NO_(x) estimated to be absorbed in the NO_(x) absorbent by the NO_(x) estimating means exceeds a predetermined allowable value.
 2. An exhaust purification device of an internal combustion engine according to claim 1, wherein said NO_(x) estimating means estimates the NO_(x) absorbed in the NO_(x) absorbent on the basis of the amount of NO_(x) discharged from the combustion chamber to the engine exhaust passage.
 3. An exhaust purification device of an internal combustion engine according to claim 2, wherein said NO_(x) estimating means is comprised of a NO_(x) calculating means for calculating the amount of NO_(x) discharged per unit time from the engine to the engine exhaust passage in accordance with the engine load and the engine rotational speed and an cumulative adding means for cumulatively adding the amounts of NO_(x) calculated by the NO_(x) calculating means.
 4. An exhaust purification device of an internal combustion engine according to claim 3, wherein said NO_(x) calculating means is provided with a memory in which is previously stored the amount of NO_(x) discharged per unit time from the engine to the engine exhaust passage as a function of the engine load and the engine rotational speed and wherein the cumulative adding means cumulatively adds the amounts of NO_(x) stored in the memory and determined from the engine load and the engine rotational speed.
 5. An exhaust purification device of an internal combustion engine according to claim 3, wherein provision is made of a throttle valve disposed in the engine intake passage for controlling the engine load and the vacuum inside the engine intake passage downstream of the throttle valve is used as a value representing the engine load.
 6. An exhaust purification device of an internal combustion engine according to claim 3, wherein an accelerator pedal is provided for controlling the engine load and the amount of depression of the accelerator pedal is used as a value representing the engine load.
 7. An exhaust purification device of an internal combustion engine according to claim 1, wherein said allowance is the maximum NO_(x) absorption capacity of the NO_(x) absorbent.
 8. An exhaust purification device of an internal combustion engine according to claim 1, wherein said allowance is a predetermined amount of absorption smaller than the maximum NO_(x) absorption capacity of the NO_(x) absorbent.
 9. An exhaust purification device of an internal combustion engine according to claim 1, wherein said allowance is a function of a temperature representing the temperature of the NO_(x) absorbent.
 10. An exhaust purification device of an internal combustion engine according to claim 9, wherein the temperature representing the temperature of the NO_(x) absorbent is the temperature of the exhaust gas.
 11. An exhaust purification device of an internal combustion engine according to claim 1, wherein said NO_(x) releasing means switches from lean to rich the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent in a predetermined time when the amount of the NO_(x) estimated by the NO_(x) estimating means exceeds the allowance.
 12. An exhaust purification device of an internal combustion engine according to claim 11, wherein said NO_(x) releasing means switches from lean to rich the air-fuel ratio of the air-fuel mixture fed to the combustion chamber for a predetermined time when the amount of NO_(x) estimated by the NO_(x) estimating means exceeds the allowance.
 13. An exhaust purification device of an internal combustion engine according to claim 11, which is provided with a reducing agent feeding means for feeding a reducing agent into the engine exhaust passage upstream of the NO_(x) absorbent and wherein said NO_(x) releasing means causes the reducing agent to be fed from said reducing agent feeding means into the engine exhaust passage upstream of the NO_(x) absorbent for a predetermined time when the amount of NO_(x) estimated by the NO_(x) estimating means exceeds the allowance.
 14. An exhaust purification device of an internal combustion engine according to claim 13, wherein said reducing agent is comprised of a hydrocarbon.
 15. An exhaust purification device of an internal combustion engine according to claim 14, wherein the hydrocarbon is at least one hydrocarbon selected from gasoline, isooctane, hexane, heptane, butane, propane, gas oil, and kerosine.
 16. An exhaust purification device of an internal combustion engine according to claim 11, wherein said NO_(x) releasing means switches from lean to rich the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent in a predetermined time during a deceleration operation when the amount of the NO_(x) estimated by the NO_(x) estimating means exceeds the allowance.
 17. An exhaust purification device of an internal combustion engine according to claim 16, wherein an intake shutoff valve which is normally fully open and is closed during deceleration operation is disposed inside the engine exhaust passage.
 18. An exhaust purification device of an internal combustion engine according to claim 11, wherein said NO_(x) estimating means makes the amount of the NO_(x) estimated to be absorbed in the NO_(x) absorbent zero when the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent is made rich during the above predetermined time.
 19. An exhaust purification device of an internal combustion engine according to claim 11, wherein said NO_(x) estimating means estimates that NO_(x) remains in the NO_(x) absorbent when the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent is made rich for a time shorter than the above predetermined time.
 20. An exhaust purification device of an internal combustion engine according to claim 19, wherein said NO_(x) estimating means estimates the amount of the NO_(x) absorbed in the NO_(x) absorbent taking into consideration the amount of NO_(x) remaining in the NO_(x) absorbent.
 21. An exhaust purification device of an internal combustion engine according to claim 19, wherein said NO_(x) estimating means causes the amount of the NO_(x) estimated to be remaining in the NO_(x) absorbent to increase the shorter the time in which the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent is made rich.
 22. An exhaust purification device of an internal combustion engine according to claim 21, wherein provision is made of a memory which stores in advance the relationship between the time during which the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent is made rich and the residual rate of the NO_(x) estimated to be remaining in the NO_(x) absorbent and wherein said NO_(x) estimating means estimates the amount of the NO_(x) remaining in the NO_(x) absorbent on the basis of the residual rate of the NO_(x) stored in the memory.
 23. An exhaust purification device of an internal combustion engine according to claim 11, wherein a NO_(x) concentration detecting means is provided for detecting the concentration of NO_(x) in the exhaust gas flowing out from the NO_(x) absorbent and a NO_(x) release promoting means is provided for promoting the releasing action of the NO_(x) from the NO_(x) absorbent when the concentration of NO_(x) is higher than a predetermined concentration when the amount of the NO_(x) estimated by the NO_(x) estimating means exceeds the allowance.
 24. An exhaust purification device of an internal combustion engine according to claim 23, wherein said NO_(x) release promoting means promotes the release action of NO_(x) by prolonging the time during which the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent is made rich.
 25. An exhaust purification device of an internal combustion engine according to claim 23, wherein said NO_(x) release promoting means promotes the release action of NO_(x) by raising the degree of richness of the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent.
 26. An exhaust purification device of an internal combustion engine according to claim 23, which is provided with a deterioration judging means for judging the NO_(x) absorbent is deteriorating when the concentration of NO_(x) is higher than a predetermined concentration when the amount of the NO_(x) estimated by said NO_(x) estimating means exceeds the allowance even when the promoting action of the release of said NO_(x) by the NO_(x) release promoting means has been performed more than a predetermined number of times.
 27. An exhaust purification device of an internal combustion engine according to claim 1, wherein said NO_(x) estimating means estimates the amount of the NO_(x) absorbed in the NO_(x) absorbent taking into consideration the reduction in the NO_(x) absorption capacity when the NO_(x) absorption capacity of the NO_(x) absorbent has fallen.
 28. An exhaust purification device of an internal combustion engine according to claim 27, wherein provision is made of a memory for storing in advance the relationship between the time in which the air-fuel ratio of the exhaust gas flowing into the NO_(x) absorbent 17 is made lean and the NO_(x) absorption capacity of the NO_(x) absorbent and wherein said NO_(x) estimating means estimates the amount of the NO_(x) absorbed in the NO_(x) absorbent on the basis of the NO_(x) absorption capacity stored in the memory.
 29. An exhaust purification device of an internal combustion engine according to claim 1, wherein an air-fuel ratio control means is provided for making the air-fuel ratio of the air-fuel mixture fed to the combustion chamber rich in accordance with the operating state of the engine regardless of whether the amount of NO_(x) estimated by the NO_(x) estimating means exceeds the allowance.
 30. An exhaust purification device of an internal combustion engine according to claim 29, wherein said air-fuel ratio control means makes the air-fuel ratio of the air-fuel mixture fed into the combustion chamber rich when the engine load is higher than a predetermined load.
 31. An exhaust purification device of an internal combustion engine according to claim 29, wherein said air-fuel ratio control means makes the air-fuel ratio of the air-fuel mixture fed to the combustion chamber rich when shifting to idling operation.
 32. An exhaust purification device of an internal combustion engine according to claim 29, wherein said air-fuel ratio control means makes the air-fuel ratio of the air-fuel mixture fed into the combustion chamber rich during an acceleration operation where the degree of acceleration is larger than a predetermined value.
 33. An exhaust purification device of an internal combustion engine according to claim 29, wherein said air-fuel ratio control means makes the air-fuel ratio of the air-fuel mixture fed into the combustion chamber rich during a shift down action of the transmission.
 34. An exhaust purification device of an internal combustion engine according to claim 29, wherein said NO_(x) estimating means makes the amount of NO_(x) estimated as being absorbed in the NO_(x) absorbent zero when the air-fuel ratio of the air-fuel mixture is made rich for more than a predetermined time.
 35. An exhaust purification device of an internal combustion engine according to claim 1, wherein the NO_(x) absorbent includes at least one substance selected from alkali metals such as potassium, sodium, lithium, and cesium, alkali earths such as barium and calcium, and rare earths such as lanthanum and yttrium and platinum.
 36. An exhaust purification device of an internal combustion engine according to claim 1, wherein the NO_(x) absorbent is comprised of a compound oxide of barium and copper.
 37. An exhaust purification device of an internal combustion engine according to claim 1, wherein a three-way catalyst is disposed in the engine exhaust passage downstream of the NO_(x) absorbent. 